24 
BULLETIN OF THE BUREAU OF FISHERIES 
attempt to rear trout on natural food alone would in most cases increase the cost 
of artificial propagation beyond all reason; but it may be possible to effect a com- 
promise whereby the fish can be given some natural food in addition to the usual 
hatchery diet. Experiments have shown that such a diet will give much better 
results than the straight heart or liver diet commonly used. 
In connection with the investigations on Octomitus we have carried on some 
experiments on the additions of vitamins to trout foods with very encouraging 
results. In these experiments cod-liver oil and dried brewer ’s yeast were added to 
the ordinary heart and liver diet to correct any deficiency of these foods in vitamins 
A and B. The results to date indicate that fish fed on a diet containing small 
percentages of oil and yeast show a greatly decreased mortality and increased 
growth over those fed a straight heart diet. 1 
While our experiments have not yet been sufficiently extensive to justify any 
general conclusions as to the effectiveness of cod-liver oil and yeast in the control 
of octomitiasis, it is believed that improvement in the food, either through the 
addition of vitamins in oil and yeast or the addition of some natural food to the 
diet, affords one of the most important methods of combating the disease. 
It has been found that the excessive loss which frequently occurred when 
infected fish were held in cans for more than 24 hours during distribution can be 
entirely prevented by avoiding overcrowding and keeping the fish at a temperature 
considerably below that at which they were formerly carried. When infected 
fish are held at temperatures of 40 to 45° F. and given an abundant supply of oxygen, 
there is very little loss, even though they may be several days in transit. 
SUMMARY 
1. Octomitus salmonis is widely distributed throughout the country and prob- 
ably occurs in all species of trout and salmon. 
2. The parasite has two phases of development within the host — one in- 
tracellular, the other extracellular. 
3. The intracellular stage is found in the epithelium of the pyloric caeca and the 
anterior end of the intestine. This stage reproduces by a process of schizogony, 
resulting in the formation of a number of merozoites, which in turn invade neigh- 
boring epithelial cells and repeat the process. 
4. After a time some of the intracellular forms cease to divide and become 
metamorphosed into the flagellated form. 
5. The extracellular or flagellated stage occurs in the lumen of the intestine and 
multiplies by a complicated process of binary fission. 
6. The flagellates form cysts, within which they divide once by binary fission. 
These cysts serve to spread the infection from one fish to another. 
7. Severe infestation by the parasite may result in serious injury to the finger- 
ling fish. Excessive numbers of the flagellates may cause the fingerlings to lose their 
appetite and become greatly emaciated. Rapid multiplication of the intracellular 
stage may result in an acute form of the disease accompanied by heavy mortality. 
i See Davis and James (1925) for further details regarding these experiments. 
